Atrioventricular (AV) synchrony, i.e., the timing of atrial and ventricular contractions, is important for good hemodynamic function in a patient. In a normal cardiac cycle, following a period of passive filling of a heart, the atria of the heart contract to squeeze blood into the ventricles of the heart. When atrial contraction is completed, the ventricles contract to eject blood into the body.
Some patients having problems with AV synchrony may have an implantable medical device (IMD), such as a pacemaker, to monitor AV synchrony and to deliver therapy to promote AV synchrony. The IMD senses atrial and ventricular events. In particular, the IMD senses atrial and ventricular activations triggered by the normal intrinsic autorhythmic activity of the heart itself. The IMD may also deliver electrical stimulations or “paces” to activate one or more atria or one or more ventricles, and thereby trigger atrial or ventricular contractions.
In a normal cardiac cycle, there is a time interval between a sensed or paced atrial activation and a ventricular activation. The time interval between a sensed or paced atrial event and an intrinsic ventricular event is called the “intrinsic AV interval,” the “spontaneous AV interval” or the “AV conduction time.” The term “intrinsic AV interval” will be used herein to refer to this time interval. In some patients, the ventricles may, from time to time, fail to activate following an atrial activation. When the ventricles fail to activate spontaneously, the IMD may deliver a pacing pulse to one or both ventricles to cause the ventricles to activate.
The IMD is programmed with a parameter known as the “AV delay” or “paced AV delay.” The parameter is also known as the “AV interval,” but the term “AV delay” will be used herein to refer to the programmable parameter and to distinguish it from the intrinsic AV interval. In general, the AV delay is the interval between a paced or sensed atrial event and a scheduled paced ventricular event. Following an atrial event, the IMD waits for the AV delay, and then paces the ventricle, unless an intrinsic ventricular event occurs prior to the expiration of the AV delay. When the IMD senses an intrinsic ventricular event, the IMD inhibits the ventricular pace for that cardiac cycle, and does not deliver the scheduled ventricular pace.
It is often desirable for the AV delay to be longer than the intrinsic AV interval. In other words, it is often desirable that intrinsic ventricular conduction be preserved, giving the ventricles an opportunity to activate of their own accord. When the ventricles fail to activate, the IMD delivers a ventricular pace. The IMD conserves power by pacing when needed, and by inhibiting pacing when the ventricles activate of their own accord.
To promote intrinsic ventricular conduction, an IMD may be programmed with “AV search” capability. AV search capability is the capacity to find the intrinsic AV interval of the heart using an AV search algorithm. In a typical conventional IMD having AV search capability, an AV search is triggered by an intrinsic ventricular activation that precedes a scheduled ventricular pace. In general, the IMD monitors a number of atrioventricular conduction sequences, monitors whether intrinsic ventricular activations are occurring before the scheduled ventricular paces, and also monitors how close in time the ventricular activations are to the scheduled ventricular paces. Using data collected during the monitoring, the IMD shortens or lengthens the AV delay by an “offset,” so that the AV delay is longer than the intrinsic AV interval.
The AV delay applied by the IMD may also be “rate adaptive,” i.e., the AV delay may automatically lengthen or shorten in response to various conditions. For example, the AV delay may shorten when the patient is engaging in physical activity, and lengthen when the patient is at rest. The amount of shortening or lengthening is called an “offset.” A rate adaptive AV delay typically lengthens or shortens without regard to whether the intrinsic AV interval lengthens or shortens in response to the conditions.
In a conventional IMD, rate adaptive operation and AV search capability are exclusive functions. If AV search capability is enabled, then the rate adaptive capability is disabled, and vice versa.